How to identify the face of a certain customer in a grocery store?
You might consider pre-training a CNN on a large dataset. The CNN should be structured such that you input 2 different images and the CNN predicts whether or not they are the same person. Your dataset should include images from multiple angles, with and without occlusions like sunglasses, and with changes in hair. (One dataset useful for this is the AR Face Database.) Then you can just check if the face matches any of the pictures you took as they entered the store.